The speaker feels shut off from everyone, and blames themselves and the people around them in equal measure. The opening image says it best: they have "locked myself from the inside out," a puzzling phrase that suggests they built their own isolation, yet can't escape it. Only they hold the key, but the poem makes clear that no one ever tried to knock.
The second stanza turns to the listener, "you," who never had time to understand or even hold the speaker's hand. The speaker insists this note is deliberate: it has been "thought out and planned." The last stanza is the darkest. Saying they've known only hate, and that by the time "you" read this it "will be too late," strongly hints at a goodbye, possibly a suicide note, though the poem never says so outright.
The mood is bitter, lonely, and quietly desperate. The steady rhymes make the voice sound controlled, which makes the pain feel even heavier.